Meanwhile, cook pasta in large pot of boiling salted water until just tender but still firm to bite, stirring occasionally. Drain. Return to pot.

Stir cheese sauce into pasta in pot. Transfer to prepared baking dish. DO AHEAD Can be made 2 hours ahead. Let stand at room temperature.

Preheat oven to 400°F. Bake pasta until cheese sauce is bubbling around edges and some ends of pasta are golden brown, 25 to 30 minutes.

Let stand 15 minutes. Serve hot.

Unlike classic mac and cheese, which is thickened by a roux (a flour and butter mixture), this one uses eggs to make a rich, gooey custard. But be careful when adding the hot cheese sauce to the eggs; you'll need to whisk the sauce in slowly so that the eggs don't curdle.